Getting back to Na_xCoO_2: spectral and thermoelectric properties

Abstract

Sodium cobaltate Na_xCoO_2 as dopable strongly correlated layered material with a triangular sublattice still poses a challenging problem in condensed matter. The intriguing interplay between lattice, charge, spin and orbital degrees of freedom leads to a complex phase diagram bounded by a nominal Mott (x=0) regime and a band-insulating (x=1) phase. By means of the charge self-consistent density functional theory (DFT) plus dynamical mean-field theory (DMFT) scheme, built on a pseudopotential framework combined with a continuous-time quantum Monte-Carlo solver, we here study the one-particle spectral function A(k,ω) as well as the thermopower S(T). The computations may account for the suppression of the e_g' pockets in A(k,ω) at lower doping in line with photoemission experiments. Enhancement of the thermopower is verified within the present elaborate multi-orbital method to treat correlated materials. In addition, the two-particle dynamic spin susceptibility χ_s(ω,q) is investigated based on a simplified tight-binding approach, yet by including vertex contributions in the DMFT linear response. Besides the identification of paramagnon branches at higher doping, a prominent high-energy antiferromagnetic mode close to x=0.67 is therewith identified in χ_s(ω,q), which can be linked to extended hopping terms on the CoO_2 sublattice.